I watched the restaurant scene from S2E6 (“Peekabo”) and the paraphrased conversation basically goes like this when it takes a turn into anger/rudeness:
Walt: You waving your checkbook around is not going to make me forget how you and Elliott, you and Elliott, cut me out.
Gretchen: That can’t be how you see it.
Walt: It was my hard work, my research, and you’ve made millions off of it.
…
Gretchen: You left me. You left me on Fourth of July weekend. When I went up to our room you were packing and wouldn’t say a word…were barely talking.
Walt: That’s your excuse, to make your empire off of my hard work?
Gretchen: How can you say that, you abandoned us. Abandoned me, Elliott.
Walt: Rich girl, just adding to your millions.
Gretchen: I don’t even know what to say to you. I don’t even know where to begin. I feel so sorry for you Walt.
Walt: Fuck you.
==========
The “our room” bit to me makes it clear Gretchen and Walt were still an item as of a Fourth of July weekend at her parent’s house. Something happens and Walt packs angrily and leaves and that by and large ends his relationship with Gretchen, Elliott, and Grey Matter. We’ll probably never know, nor do we really need to know, what happened there. We can assume Walt was probably slighted in some way, and the insane pride and ego that we know exists exploded out of him. For most of his life he kept that part of himself under wraps, especially during his 16-17 years of “normal” married life with Skyler. But it was always there, and it caused him problems long ago. It’s also probably why he later went from doing lab work as a real chemist to being a high school chemistry teacher.
It’s obviously what lead to him making meth (his pride prevented him from taking a handout from Gretchen and Elliott) and what lead him to continue making it even when there were multiple times he could have just simply quit throughout this and his family would have been safe. He even could have quit prior to Skyler finding out about the meth and probably still would have had the ability to leave them a fortune.
Hell, he was free and clear until he told Hank that the notebook was the work of someone copying a mentor or whatever. Hank is smart, but he was very, very content with Gale as Heisenberg. He was done, finished, with that until Walt opened his mouth.
